The invention relates to a bus controlled arrangement for a video apparatus. In particular, the invention relates to an arrangement for adjusting a current in a winding mounted on a cathode ray tube (CRT) to compensate for the earth's magnetic field.
U.S. Pat. No. 5,168,195, entitled, MAGNETIC FIELD COMPENSATION APPARATUS, in the names of Breidigan, et al. describes a compensating coil for a scanning electron beam display, such as a color television tube, that reduces undesirable deflection of the electron beams due to ambient magnetic fields, in particular the geomagnetic field. The coil has a winding disposed to encircle the tube neck, perpendicular to the Z axis. A degaussing coil is positioned on the tube envelope to provide for demagnetizing metal structures within the envelope. In order to provide more complete degaussing, the supply current to the compensating coil is interrupted during the degaussing operation.
U.S. Pat. No. 5,739,638, in the name of Wilber et al., entitled Bus controlled arrangement using a duty cycle modulated control signal in a CRT, describes an arrangement using a microprocessor that applies a digitally coded signal to a digital-to-analog (D/A) converter. An output voltage of the D/A converter is applied via a power amplifier to the compensating coil.
In the power amplifier, a differential current sense arrangement is used to control a voltage developed across a current sense resistor that is coupled in series with the compensating coil. Thereby, the current in the compensating coil is made independent of the value of the resistance of the compensating coil. This allows Z coils with significantly differing resistances to achieve identical rotation ranges.
In carrying out an inventive feature, a pair of differential output signals of a corresponding pair of DAC's fabricated in an integrated circuit (IC) on a common substrate are coupled to corresponding inverting and non-inverting inputs of the power amplifier. Such arrangement facilitates tracking between the pair of differential output signals.
During a degaussing interval, the pair of DAC's are programmed to convert the same digital value, for example a mid-range value. Because of the tracking between the pair of differential output signals, advantageously, accurate zero current is produced in the compensating coil, during the degaussing interval. Outside the degaussing interval, one DAC output signal may remain at the mid-range value while the other one may be adjusted to either a higher or a lower value as required for the aforementioned earth magnetic field compensation.